This serial number is usually the only feature unique to an individual banknote, but seldom has any high security value in itself, although it generally comprises a unique typeface. When a forgery is discovered, the serial number is frequently circulated to warn the public that notes bearing this number are suspect. In order to circumvent this the forger may utter forgeries with different serial numbers and the more the form of the characters differs from that of conventional characters the more difficulty he will have in obtaining a suitable serial numbering device. An alternative method of forgery is to photograph the numbers on a note and print a series of forgeries bearing this serial number by conventional methods. Changing the serial number will then involve a separate photographic and plate-making process for each serial number used and the more difficult this process the slower and more time consuming it is to produce forgeries with differing serial numbers.
The security means disclosed herein are therefore intended to make the serial numbers a positive security feature, by making them unique in style and difficult to print except by a specially manufactured machine.